I was rather confused by the dictionary comprehension syntax used there, because I wasn't aware that you could write one without the ":" to delineate the key: value pair. Turns out you can, but it just creates a dict with no values stored, just the keys! This works here because the returned dict is an iterable that returns the keys on iteration, and "update" accepts an iterable of (key, value) tuples - and the keys are just that in this case. So the effect is the same as if it was a list comprehension! Just slightly more confusing